When metal surfaces, particularly aluminum surfaces, are exposed to hot aqueous solutions for extended periods of time, there is a tendency for such surfaces to develop a stain which may range to a brown or black. Discoloration of the aluminum surfaces becomes a problem in certain industries, particularly in the food industry. For example, alcoholic beverages are pasteurized in metal containers by subjecting the cans to hot water baths or sprays in the range of from about 110.degree. F. to 170.degree. F. When the metal containers are subjected to hot water, there is a tendency with the metal surface, particularly in aluminum surface, to stain upon exposure to the atmosphere. One technique which has been utilized to prevent tarnishing is the application of a conversion coating to metal containers. Chromates and phosphates have been utilized in the industry as conversion coatings for inhibiting corrosion.
In the manufacture of aluminum cans, the cans have been washed with acidic cleaners to remove aluminum fines and other contaminants. Concern regarding the residue remaining on the cans following acidic cleaning (e.g., fluoride) has resulted in the evaluation of alkaline cleaning procedures for removing such fines and contaminants. However, the aluminum cans which have been cleaned with alkaline cleaning solutions are still subject to discoloration such as by the formation of brown stains. Furthermore, darker stains often result from linestops, which occur frequently in high-speed container washers. These stains are aesthetically unacceptable and may result in the rejection or scrapping of the final product, thereby increasing manufacturing costs.
A clean and stain-free aluminum surface also is desirable in order to insure the proper application of paints and inks. It is also desirable that the aluminum cans can be conveyed through printers at high speed. The term "mobility" is used in the industry to refer to the ability of an aluminum container to travel smoothly through the manufacturing process conducted at the highest speed possible. Improved mobility allows for increases in production and increased profits. If the containers are not characterized by an acceptable mobility, the flow of cans through the printers is affected and often results in frequent jammings, down time, printer misfeeding problems, loss of production and high rate of can rejects.
U.S. Pat. Nos. 4,341,878 and 4,351,883 describe compositions and processes for treating aluminum surfaces for tarnish and corrosion resistance. The process involves contacting the aluminum surfaces with an aqueous solution containing a mixture of an alkali metal silicate and an organic polymer having displaceable hydrogens or displaced hydrogen. Typical of the organic polymers disclosed in these patents are polyacrylates, polyvinyl alcohols, polystyrene sulfonic acid, etc.
U.S. Pat. No. 4,457,322 describes alkaline cleaning compositions for aluminum surfaces which are non-corrosive and which avoid discoloration or tarnishing of aluminum surfaces. The compositions comprise a mixture of an alkali metal metasilicate and a compound selected from sodium carbonate, potassium carbonate, lithium carbonate, potassium orthophosphate and sodium orthophosphate. The compositions may also include surfactants selected from nonionic, anionic, amphoteric or zwitterionic detergents.
U.S. Pat. No. 4,599,116 describes an alkaline cleaning process for aluminum container surfaces. The aqueous alkaline cleaning composition contains an alkalinity agent, a complexing agent to dissolve at least some of the metal ions removed from the metal surface by the cleaning solution, and at least one surfactant to remove organic soils from the surfaces of the container and to inhibit white-etch staining of the surfaces. Examples of complexing agents include gluconic acid, citric acid, tartaric acid, sodium tripolyphosphate, etc.
U.S. Pat. No. 4,859,351 describes a lubricant and surface conditioner for formed metal surfaces such as aluminum cans. The composition is stated to reduce the coefficient of static friction on the outside surface of the cans which permits a substantial increase in production line speed. The lubricant and surface conditioners disclosed in this patent are selected from water-soluble organic phosphate esters; alcohols; fatty acids including mono-, di-, tri-, and poly-acids; fatty acid derivatives such as salts, hydroxy acids, amides, esters, ethers and derivatives thereof; and mixtures thereof. The lubricant and surface conditioner may be applied to the cans during the wash cycle, during one of the treatment cycles, or after the final water rinse.